Connected: An Internet Encyclopedia
RFC 1035 - 4.1.1. Header section format

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### 4.1.1. Header section format

The header contains the following fields:

                                        1  1  1  1  1  1
          0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
        |                      ID                       |
        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
        |QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   |
        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
        |                    QDCOUNT                    |
        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
        |                    ANCOUNT                    |
        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
        |                    NSCOUNT                    |
        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
        |                    ARCOUNT                    |
        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where:

    ID              A 16 bit identifier assigned by the program that
                    generates any kind of query.  This identifier is copied
                    the corresponding reply and can be used by the requester
                    to match up replies to outstanding queries.

    QR              A one bit field that specifies whether this message is a
                    query (0), or a response (1).

    OPCODE          A four bit field that specifies kind of query in this
                    message.  This value is set by the originator of a query
                    and copied into the response.  The values are:

                    0               a standard query (QUERY)

                    1               an inverse query (IQUERY)

                    2               a server status request (STATUS)

                    3-15            reserved for future use

    AA              Authoritative Answer - this bit is valid in responses,
                    and specifies that the responding name server is an
                    authority for the domain name in question section.

                    Note that the contents of the answer section may have
                    multiple owner names because of aliases.  The AA bit
                    corresponds to the name which matches the query name, or
                    the first owner name in the answer section.

    TC              TrunCation - specifies that this message was truncated
                    due to length greater than that permitted on the
                    transmission channel.

    RD              Recursion Desired - this bit may be set in a query and
                    is copied into the response.  If RD is set, it directs
                    the name server to pursue the query recursively.
                    Recursive query support is optional.

    RA              Recursion Available - this be is set or cleared in a
                    response, and denotes whether recursive query support is
                    available in the name server.

    Z               Reserved for future use.  Must be zero in all queries
                    and responses.

    RCODE           Response code - this 4 bit field is set as part of
                    responses.  The values have the following
                    interpretation:

                    0               No error condition

                    1               Format error - The name server was
                                    unable to interpret the query.

                    2               Server failure - The name server was
                                    unable to process this query due to a
                                    problem with the name server.

                    3               Name Error - Meaningful only for
                                    responses from an authoritative name
                                    server, this code signifies that the
                                    domain name referenced in the query does
                                    not exist.

                    4               Not Implemented - The name server does
                                    not support the requested kind of query.

                    5               Refused - The name server refuses to
                                    perform the specified operation for
                                    policy reasons.  For example, a name
                                    server may not wish to provide the
                                    information to the particular requester,
                                    or a name server may not wish to perform
                                    a particular operation (e.g., zone
                                    transfer) for particular data.

                    6-15            Reserved for future use.

    QDCOUNT         an unsigned 16 bit integer specifying the number of
                    entries in the question section.

    ANCOUNT         an unsigned 16 bit integer specifying the number of
                    resource records in the answer section.

    NSCOUNT         an unsigned 16 bit integer specifying the number of name
                    server resource records in the authority records
                    section.

    ARCOUNT         an unsigned 16 bit integer specifying the number of
                    resource records in the additional records section.

Next: 4.1.2. Question section format

Connected: An Internet Encyclopedia
RFC 1035 - 4.1.1. Header section format